1. Field
The present disclosure is related generally to power supplies that requires isolation, and more particularly, to uninterruptible power supplies configured to prevent surges and excessive radio frequency interference (RFI) and electromagnetic interference (EMI).
2. Description of the Related Art
The use of electronic and computer systems has expanded immensely and affects almost all ways of life. Such systems have also been adopted for medical use and their presence is ubiquitous. Even brief interruptions of or variations in a main input power supply can cause failures to these systems, resulting in lost time, data, damaged equipment, and high repair bills. To overcome these problems, uninterruptible power supply (UPS) systems have been developed and are well known in the art. These include both on-line and off-line configurations. The simplest type, known as ‘single conversion’ or ‘standby’, uses a battery, an inverter to convert the battery voltage to an AC voltage, and a transfer switch that switches the UPS output from the main supply to the inverter output if the main supply voltage is lost. The transfer time from the main power supply to the UPS power supply during the switching can be noticeable and can become a significant problem in some computer applications. A sharply reduced transfer time is desirable.
An alternate type of UPS known as ‘double conversion’ or ‘on-line’ eliminates this transfer time. The main AC supply voltage is continuously converted by a rectifier to a DC voltage and charges a battery that is in parallel with the DC voltage. This DC voltage is converted by an inverter back to an AC output voltage. If the AC supply voltage is lost, the battery supplies the DC voltage to the inverter immediately and there is no interruption in power to the load. Upon the return of the main AC supply voltage, the load is again supplied by the main AC power supply without any interruption in power. Since the incoming power is rectified, incoming power is inherently conditioned since surges, sags, and noise can be essentially eliminated during the conversion to DC.
Many types of on-line UPS systems have been developed including a three port transformer having a first primary winding connected to an AC input, a second primary winding connected to a battery-inverter arrangement, and the secondary winding connected to the load. A series regulator is used to keep the output voltage within certain limits when the main AC supply input is present. A synchronization circuit provides a means of having the AC output of the inverter in phase with the AC input when a transfer takes place to prevent large voltage transients. Phase lock loops (PLL) have been commonly used as a means for locking one frequency to another to provide line synchronization and are commercially available as integrated circuit packages, such as a Harris Semiconductor type CD4046B. Another UPS system adaptable for use with alternate energy supplies requires an input supply of given frequency and amplitude. A DC bus is established that is fed from either rectified AC input power or batteries. The DC bus voltage is converted back to an AC output voltage. Various configurations using different combinations of AC/DC, DC/AC, DC/DC, and AC/AC converters have been used. Regulation of the AC output is controlled by keeping the DC bus at a predetermined voltage level. Energy transfer into the bus is controlled by adjusting the magnitude and phase relationship of the AC input voltage and the AC side of an AC/DC converter feeding this bus.
These and other known types of UPS devices have various operational characteristics and features that are unique to the method employed and would be difficult to integrate into a single, cost-effective device. Features desired for a UPS system include high efficiency in the normal mode of operation, small size, reliability of components, regulation of the AC output voltage during both on-line and standby modes of operation, fast transfer times between modes with low electrical noise generation, extended battery life, line isolation between supply and load, elimination of voltage spikes to the load and the device during transfer times, and diagnostic capabilities.
A microprocessor-based UPS device allows the integration of these features wherein the microprocessor reduces the need for extensive hardware, with a reduction in power requirements, without compromising performance while increasing the overall efficiency of the UPS device. However, most output inverter stages employed in UPS systems compare the voltage output with a desired output in an error amplifier to produce an error signal proportional to the error. The error signal is then applied to the input of a pulse width modulator (PWM) operating at a frequency much higher than the output frequency. The width of the output pulse is modulated with respect to the error signal and applied to a switching type amplifier. The output is filtered by an inductor to remove the high frequency components caused by the switching mode of operation. This may cause a problem in system frequency response and stability since the inductor introduces a pole in the system transfer function. To overcome this problem, an inner current feedback control loop is introduced to the voltage feedback to effectively eliminate the output inductor. These types of feedback systems utilize peak current mode control. This control requires slope compensation in the circuitry and requires compensation for peak to average current errors. To improve on this drawback, a current feedback system that relies on average current rather than peak current is desired. These types of control systems have been commonly developed for DC amplifiers but do not exist for AC systems.
Another concern commonly associated with UPS systems is providing adequate protection for the solid state output devices to prevent failures due to overloads, short circuits, and overheating. Some current power supplies control the power conversion across the transformers with independent analog loops. Because they are independent, the relative phase between the loops is not controlled. If two transformers are turned on simultaneously and are in phase, a current spike will be produced that is much larger than desired. Controlling the transformers so that they are out of phase is desirable.
Many UPSs will not start up without charged batteries installed and on line, since the control circuits are powered solely by the batteries. Although this is a valuable feature in some applications, this imposes a severe limit on usefulness when the batteries are uncharged but are present and can be charged while the UPS is being used.
Similarly, many UPSs will not start up on battery power, even when the batteries are fully charged, when there is no AC input power available. This imposes a limitation on the operation of equipment connected to the UPS after AC power has been lost, which is when a UPS would be of the greatest benefit.